This invention relates to a multi-beam laser diode used in a raster output scanner and, more particularly, to a raster output scanner which uses a deflection wedge to control the laser diode beam separation.
Referring to FIG. 1, a sagittal view of a conventional multi-beam raster output scanner is shown which utilizes a multi channel laser diode 10, a collimating lens 18, a spot size control aperture 20, a Fourier Transform/focusing lens 28 and a scanning polygon (not shown) and a photoreceptor plane P. Hereinafter, Fourier Transform/focusing lens 28 is referred to as focusing lens 28. In order to collimate each beam 12, coming from the laser diode 10, the collimating lens 18 is placed one focal length f.sub.1 away from the laser diode 10. The collimating lens 18 receives several beams 12 from different channels of the laser diodes 10. The collimated light beams emerging from the collimating lens 18 start converging towards each other and they all cross each other at the focal point 19. The spot size control aperture 20 is placed at the focal plane for clipping the light. After passing through the spot size control aperture 20, the collimated beams 12, which have crossed each other at the focal point 19, start diverging from each other. The focusing lens 28 is placed at one focal length f.sub.2 away from the spot size aperture 20. After passing through the focusing lens 28 and through the standard imaging optics (not shown), the beams 12 individually focus to a spot with a spot size 38 on the photoreceptor plane P. At the photoreceptor plane P, there is a distance 36 between the centers of adjacent spots and hereafter is referred to as spot separation.
In multi-beam scanning systems, the spot separation 36 relative to the spot size 38 is an important element. It should be noted that the spot separation 36 is the distance between the center rays of adjacent light beams 12. Simultaneous scanning of multiple spots requires a selection of proper spot separation to spot size at the photoreceptor plane. In a multi channel laser diode, there is a limit in placing the channels close to each other. If the channels are placed too close to each other, then the problem of cross-talk arises. Cross-talk happens when the information from one channel leaks into the adjacent channel due to closeness of the channels. To reduce the cross-talk problem, the channels separation on the multi channel laser diode is increased which in turn generates a larger spot separation. Therefore, to achieve the proper spot separation 36 to the spot size 38, the spot separation 36 or the spot size 38 should be modified. To modify the relative spot separation 36 to the spot size 38, either the spot separation has to be decreased or the spot size has to be increased.
In the conventional multi-beam raster output scanner, shown in FIG. 1, only the spot size can be modified. By changing the size of the control aperture 20, the spot size can be changed. The spot size has an inverse relationship with the size of the control aperture 20. Normally, the relative spot separation to the spot size is too high and therefore the spot size has to be increased. For example, in order to double the spot size, the control aperture 20 should clip the beams at 63% of their diameter which results in 20% loss of intensity. Moreover, there is no element in the design of the conventional multi-beam raster output scanners which can modify the spot separation independently. Different optics used in the raster output scanners change the spot separation and the spot size at the same rate. Therefore, the relative spot size to spot separation stays the same. Since there are no means to modify the spot separation independent of the spot size, in order to achieve the proper spot separation 36 relative to the spot size 38, the only option is to clip the radius of the beams at the price of losing intensity.